This invention relates to motor vehicle anti-lock brake systems, and more particularly to a control method that adaptively determines an initial brake pressure reduction to be initiated upon detection of insipient wheel lock.
Motor vehicle anti-lock braking systems modulate hydraulic brake pressures upon detection of insipient wheel lock to maximize the tractive force between the vehicle tires and the road surface. The brake pressure is initially reduced by an amount based on an assumed coefficient of friction between the tires and road surface, and then re-applied once the wheel acceleration reaches a reference acceleration value. Since the coefficient of friction is ordinarily unknown, the initial pressure reduction is typically determined for a worst-case road surface, such as glare ice, even though this approach provides somewhat degraded braking performance on surfaces having a higher coefficient of friction. After the initial pressure reduction, the coefficient of friction is inferred based on the time required for the wheels to accelerate to the reference value, and the rate of pressure re-application is scheduled accordingly. Although the coefficient of friction can be initially estimated from the brake pressure or the brake pedal force, the sensors for obtaining such information significantly increase system cost, and the estimated coefficient of friction is subject to error under conditions where the relationship between brake pressure and brake torque deviates from the norm. Accordingly, what is needed is an inexpensive and reliable method of determining an initial brake pressure reduction that optimizes the performance of an anti-lock braking system.
The present invention is directed to an improved anti-lock brake control method in which the brake pressure reduction initiated upon detection of insipient wheel lock is adaptively determined based on a periodically updated characterization of the relationship between brake pedal position and vehicle deceleration. In a preferred embodiment of the invention, the brake torque and vehicle weight are estimated based on the characterization data, corrected for variation in brake heating, and used to compute the surface coefficient of friction, which in turn is used to schedule the initial anti-lock brake pressure reduction for optimal braking system performance.